Wolbachia Lateral Gene Transfer
Lateral, or horizontal gene transfer, is the exchange of genetic material between organisms (1). This process has been suggested to be an important mechanism by which species evolve (2). However it wasn't until drug resistant bacteria began appearing world wide in the 1950's that the significance of lateral gene transfer had on species evolution begin to be fully appreciated (3). Lateral gene transfer has lead to the evolution many traits in prokaryotes including antibiotic resistance, alterations in virulence, as well as enhanced metabolic capacities (2). Prokaryote to eukaryote lateral transfer has been observed as well. The most well know examples of this involve endosymbioitic gene transfer (4). This occurs when a eukaryote engulfs another cell which is then retained as an endosymbiont. These relationships can be transient or can be become obligate (4). Endosymbiotic relationships allow for the transfer of genes from one organism to another. The mitochondria and plastids are examples of organelles which developed from endosymbiotic relationships. Most of the proteins required by these organelles to function are actually encoded by nuclear genes rather than genes located in the organelles genome. This suggests that endosymbiotic gene transfer has occurred (5, 6). Wolbachia, a common intracellular bacterial endosymbiont, has been shown to have had large tracts of its genome transferred to its insect and nematode hosts (7). However the functions of these transfers are not yet clear. Wolbachia and Lateral Gene Transfer Few lateral gene transfer events involving higher eukaryotes have been identified. However, Wolbachia pipientis, ''a maternally inherited endosymbiont which infects a wide range of insects and nematodes (7), has been show to be involved in gene transfer to higher eukaryotes. Initially ''Wolbachia host transfer was identified in the insect Callosobruchus chinensis, commonly known as the adzuki bean beetle (8). Callosobruchus chinensis ''was believed to be infected by three different strains of ''Wolbachia; wBruCon, wBruOri, and wBruAus which were all distinguishable by their surface protein gene sequences. These strains are inherited only in the maternal linage. Thus, there is a close link to the life cycle of Wolbachia and the germ cell lines of the host (8). In a study conducted by Kando et. al. Callosobruchus chinensis were feed beans containing tetracycline or rifampicin in order to eliminate all Wolbachia. However they could not eliminate the wBruAus strain. DNA of the beetles was also digested using the restriction enzymes Bam''HI of ''Hind''III. The fragments were then subject to PCR using ''Wolbachia specific primers. The PCR amplified fragments were then inserted into plasmids and sequenced. This revealed the presences of bacterial genes but also lead to the identification of non-LTR retrotran sposon like sequences. These retrotransposon like sequences were highly similar to transposable elements found in Drosphilia. The researchers believed this was evidence that lateral gene transfer had occurred. Furthermore, breeding experiments using beetles that were infected with the wBruCon and wBruOri strains showed maternal inheritance of bacteria as anticipated but beetles that were only wBruAus positive showed an X chromosome linked pattern of inheritance (Figure 1). Additionally, females showed twice as much wBruAus as males as determined by Wolbachia surface protein (wsp) gene copy number (Figure 2) further s uggesting X linked inheritance. As a whole the researchers believed that the antibiotic resistance, the presences of transposable elements in the amplified bacterial DNA, and the X linked pattern of inheritance suggested that wBruAus was not an actual bacterial entity but a fragment of Wolbachia genome that had been transferred to Callosobruchus chinensis. Numerous mutations were in the transferred DNA and it was not believed to be expressed (8). Wolbachia gene transfer has also been found to have occurred in the nematode, Onchocerca volvulus. In a study conducted by Fenn et. al. over 70kb of wOvo, Wolbachia from Onchocerca volvulus, genome was sequenced. Comparison of the sequenced segment to sequenced regions of the genome of Onchocerca volvulus revealed significant sequence similarity with two wOvo genes, OWJ-2 ( a predicted phosphomannomutase) and OW4-C. In order verify the findings another source of Onchocerca volvulus and orthologous insertions were identified in the genome of the related cattle parasite Onchocerca ochengi (Figure 3). The transferred DNA were not complete genes and had accumulated numerous mutations (9). Lateral gene transfer has also been found to have occurred between Wolbachia and Drosophilia ananassae in which nearly the entire genome of Wolbachia has been transferred (7). Furthermore lateral gene transfer between Wolbachia and the nematode Brugia malayi has been identified (7). The discovery of lateral gene transfer between Wolbachia and its hosts indicate that gene transfer can occur in higher eukaryotes. These events may occur through endosymbiotic relationships. However, due to the separation of gametes in animals the transferred DNA may not be passed from generation to generation but could still influence the functioning of the host organism (10). Additionally, somatic animal cells that continue to propagate could pass on the transferred material throughout the hosts lifetime (10). Sources 1. Andersson J.O. Lateral gene transfer in eukaryotes. Cellular and Molecular Life Sciences 2005; 62: 1182-1197. 2. Ochman H, Lawrence JG, Groisman EA. Lateral gene transfer and the nature of bacterial innovation. Nature 2000; 405: 299-304. 3. Davies J. Origins and evolution of antibiotic resistance. Microbiologia 1996; 12: 9-16. 4. Keeling PJ, Palmer JD. Horizontal gene transfer in eukaryotic evolution. Nature Reviews 2008; 9: 605-618. 5. Martin W, et. al. Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus. PNAS 2002; 99: 12246-12251. 6. Kurland CG, Andersson SG. Origin and evolution of the mitochondrial proteome. Microbiol. Mol. Biol Rev. 2000; 64: 786-820. 7. Hotopp et al. Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science 2007; 317(5845): 1735-1756. 8. Kondo N, Nikoh N, Ijichi N, Shimada M, Fukatsu T. Genome fragment of Wolbachia endosymbiont transferred to X chromosome of host insect. PNAS 2002; 99(22); 14280-14285. 9. Fenn K, Conlon C, Jones M, Quail MA, Holroyd NE, Parkhill J, Blaxter M. Phylogenetic relationships of the wolbachia of nematodes and arthropods. PLoS Pathogens 2006; 2: e94. 10. Robinson KM, Sieber KB, Hotopp JCD. A reveiw of bacteria-animal lateral gene transfer may inform our understanding of diseases like cancer. PLoS Genetics 2013; 9(10): e1003877.